(1) Field of the Invention
The present invention relates to the field of semiconductor fabrication. More specifically, the present invention relates to integrated circuits that include structures that reduce or prevent damages to the integrated circuit.
(2) Prior Art
FIG. 1 shows a cross-sectional view through a semiconductor wafer 100. Semiconductor wafer 100 includes a silicon substrate 101 that has several dielectric layers 102, 103 and 104 formed thereupon by processes that are well-known in the art. The figure also shows three metal layers that have been deposited, masked, and etched to form metal layers 105, 106, and 107. Metal layers 105-107 overlie device regions to connect the various devices and the subsequently deposited metal layers. A top dielectric layer 111 (passivation layer) is then formed over the terminal metal layer 105 to planarize, insulate the electrical devices and interconnects, to prevent leakage of electrical current through the passivation layer, etc. The passivation layer 111 may include a hard passivation layer 113 typically made of silicon nitride (SiN) and a soft passivation layer typically made of polyimide 115. The polyimide layer covers the silicon/nitride layer. The soft passivation layer is deposited by Plasma Enhanced Chemical Vapor Deposition or other typically known passivation layer deposition processes. The layer of silicon nitride is then covered by polyimide or another type of soft passivation layer.
FIG. 2(a) illustrates a cross-section through the semiconductor wafer of FIG. 1 where the soft passivation layer 115 is partly delaminated from the hard passivation layer 113. The delamination typically allows moisture and other impurities to penetrate in the semiconductor wafer. One reason for the passivation layer's delamination is that the die is assembled into a plastic package which is more conducive to propagation of external forces within the package and to the die. The passivation layer may also delaminate when the die and the passivation layer are subjected to pressure pot testing (steam at a pressure greater than atmospheric pressure). One theory explaining this type of delamination is that steam dissolves or weakens the bonds within the passivation layer causing the passivation layer itself to delaminate.
Delamination is more likely to occur at the interface between the passivation layer 111 and the terminal metal layer 105. Delamination typically starts at the edge of the die and propagates towards the center of the die. If the delamination reaches an electrical interconnect, the forces within the die that cause the delamination are applied to the electrical interconnect causing the electrical interconnect to rip apart at weak points thereof.
FIG. 2(b) illustrates another cross section of the semiconductor wafer of FIG. 1 where the polyimide layer 115 delaminates together with the silicon nitride layer 113.
FIG. 2(c) illustrates a cross section of the semiconductor wafer of FIG. 1 where the terminal metal layer 105 and the passivation layer 111, including the silicon nitride layer 113 and the polyimide layer 115, delaminate from dielectric layer 104 as a result of external forces applied to the semiconductor wafer of FIG. 1.
In certain integrated circuits, the terminal metal layer TML includes a continuous guard ring that surrounds a die active area of the integrated circuit. The guard ring protects the die active area from damages. Damages, among other things, include invasion by foreign impurities, such as sodium and magnesium, that exist in the environment, and certain mechanical damages, including micro-cracks that may be produced when a wafer is cut into dice. Micro-cracks propagate to die active areas of the chips producing damages thereto.
FIG. 3 illustrates a top view of a die 300 that has a guard ring 304 surrounding the die active area 301 of the integrated circuit. Guard ring 304 protects the die active area from damages.
Present state of the art guard rings are not robust enough to withstand the various forces exerted on the IC. The guard rings may get broken during reliability testing, specifically during temperature cycling to which the integrated circuit is subjected. Shear forces may be exerted to the guard ring during temperature cycling causing damages particularly at and near the corners of the guard ring where these forces have a more destructive effect.
FIG. 4 illustrates a cross-sectional view through a part of a guard wall including guard ring 404 formed in a terminal metal layer (TML) (Metal5). A passivation layer including a conformal nitride layer 406 is formed over the TML including guard ring 404. The nitride layer 406 formed over guard ring 404 has a re-entrant wall 410 that forms a re-entrant angle 408 with a surface 412 of the layer of nitride deposited over the terminal layer of interlayer dielectric (ILD4) 416. The Metal5 guard ring 404 is coupled to a Metal4 guard ring, by way of via 418. During temperature cycling, guard ring 404 tends to move sideways due to the expansion and contraction of the metal of which the guard ring is made and to the shear forces 419 exerted upon the re-entrant wall 410. Moreover, due to forces 419 exerted upon re-entrant wall and the fact that the portion of nitride in the vicinity of the re-entrant angle 408 is thinner, moisture may penetrate into the corner defined by re-entrant angle 408, causing the nitride to crack. This may cause exposure of the Metal5 guard ring 404 to outside agents that may damage the Metal5 guard ring 404.
The fact that the Metal5 guard ring 404 protrudes outside causing the nitride layer to have the profile shown in FIG. 4 (with the re-entrant wall 410 and re-entrant angle 408) causes problems at the interface between the Metal5 and the terminal dielectric layer 416. Moreover, guard ring 404 contributes to instability in the guard wall including guard ring 404 and the other layers of metal as guard ring 404 is subjected to high forces (shear forces). It is desirable to provide a guard wall that is stable and does not contribute to the type of damages caused to the nitride layer and to the guard wall explained above.